An aircraft's air data system measures outside air pressure to provide, for example, airspeed and altitude data to cockpit instruments.
Typically, air data systems use pressure sensors comprising forward (pitot) and side-facing (static) orifices on the surface of the aircraft. These orifices are linked to cockpit gauges, or pressure transducers, via small-diameter pneumatic tubing.
An air data system may also measure angle of attack and sideslip. This may be used to provide stall warning, improved turn coordination, even stability augmentation etc.
Typically, angle of attack and sideslip are measured using airstream direction detectors (e.g. self-aligning vanes).
Alternatively, pressure and flow angle measurement capability may be combined in a single multi-function unit that integrates sensors, transducers, and software, to convert analogue measurements to calibrated digital output data. Multi-function probes may be self-aligning, or fixed designs that derive both pressure and flow angle data purely as a function of local measured absolute and differential pressures.
Generally, accurate, and redundant, air data is important for flight control and operation in controlled airspace. Air data accuracy may be achieved through the “optimal” location of air data sensors on the aircraft so as to minimise aircraft aerodynamic interference effects on air data. Multiple redundant sensors and transducers may be used to achieve reliability.
Typically, air data sensors are mounted on the aircraft forebody. However, achieving optimal locations of multiple sensors (even when multi-function air data probes are used) tends to be difficult due to the competition for space in the nose of the aircraft with other equipment and sensors. Generally, non-optimal air data sensor locations demand more complex correction algorithms and tighter accuracy and repeatability tolerances in order to meet output parameter accuracy requirements.
Also, NASA developed a nose-mounted flush air data sensing (FADS) system. This system comprised an array of pressure tappings, transducers with off-aircraft modelled correction algorithms. Further information on this system can be found in NASA Technical Memorandum 104241, “Development of A Pneumatic High-Angle-of-Attack Flush Airdata Sensing (HI-FADS) System”, S. A. Whitmore, November 1991.
NASA has also investigated the feasibility of mounting surface pressure tappings on the wings of an aircraft—potentially using the air data correction algorithms developed for the HI-FADS system. Further information can be found in NASA Technical Memorandum 104267, “Application of a Flush Airdata Sensing System to a Wing Leading Edge”, S. A. Whitmore et al. February 1993.
Separately to mounting air pressure sensors on aircraft wings, U.S. Pat. No. 6,550,344 discloses a semi-flush air data sensing probe formed as an elongated bubble housing supported on a support surface. The sensing probe has a generally longitudinally extending rounded outer edge surface with a rounded contoured leading end. A forwardly facing port is provided at the leading end and centred on a central plane. A pair of angle of attack sensing ports is also provided on the leading end, these ports are spaced from one another and are symmetrically located on opposite sides of the central plane.